×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Overexcavation depth on expansive shales.

Overexcavation depth on expansive shales.

Overexcavation depth on expansive shales.

(OP)
Is there a typical depth of overexcavation and fill replacement under foundations on expansive shales?

RE: Overexcavation depth on expansive shales.

Typical?  Not really.  

It will depend upon how expansive the soil/shale is, how you are loading it and what the structure is.

First step should be several lab tests to determine how expansive the soil is.

RE: Overexcavation depth on expansive shales.

(OP)
Single story residence, swell is about 11% with 60 psf surcharge.

RE: Overexcavation depth on expansive shales.

What is the psi needed to prevent swell?

RE: Overexcavation depth on expansive shales.

I am curious about your diagnosis of the reason the shale is swelling. Stress relief after excavation? Expansion of montmorillonite due to water intrusion? Pyrite alteration?  We have seen some heaving of shale in Alabama and am interested in what folks are experiencing in other parts of the country.  

RE: Overexcavation depth on expansive shales.

11 % swell is extrememly high, we would typically recommend drilled piers and structural floors on a site with that high of a swell.  Deep overexcavations of 8 to 10 feet have also been used.  The zone of overexcavation needs to extend one to one form the perimeter of the building.

RE: Overexcavation depth on expansive shales.

Shales here (Ottawa, Ontario Canada) are expansive from pyritic alteration of limestone to gypsum.  Greatest uplift pressure I have seen reported is 50 or 70 kPa, but this is only from recollection, and is typically only a problem under slabs on grade, or walls cast directly against it.

The conditions needs to be "right" for this bio-chemical reaction to occur.  It can be mitigated by keeping the shale away from air, permanent submergence works if you can deal with the other issues associated with it.  Sealing with bitumens has been tried but seems to be less than permanent.  The reaction is worse under warm areas, like boiler rooms.

Drilled piles and a structural basement slab, with an expansion buffer is another successful strategy.

Another is to build a raft stiff enough to carry the swell load back to the columns, sort of an upside down uniform load on a two way column supported slab.   

RE: Overexcavation depth on expansive shales.

soils 212 hit the nail on the head - 50 to 70 kpa needs 8 to 10 feet of overburden to hold it down.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources